1. Field of the Invention
The present invention relates to injection-molded articles having excellent heat stability and mechanical strength which are prepared by using aliphatic polyesters with biodegradability and sufficiently high molecular weights and specific melt properties for practical use.
2. Discussion of the Background
Recently, with the advancement of plastics in industrial parts, automotive members, domestic articles and the like, the resulting waste of the large amount of plastics used in these materials cause pollution of rivers, oceans, soil and the like and is becoming a great social problem. To prevent such pollution the development of biodegradable plastics has been desired; for example, poly(3-hydroxybutylate) produced by fermentation methods using microorganisms, blends of general-purpose plastics and starch, a naturally occurring polymer, and the like are already known. The former polymer has a drawback in that it is poor in molding properties because the polymer has a heat decomposition temperature close to its melting point and a raw material efficiency is very bad because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not by itself have thermoplasticity, the polymer has defects in molding properties, and is greatly limited in its range of application.
On the other hand, although it is known that aliphatic polyesters are biodegradable, they have hardly been used because polymeric material sufficient enough to obtain practical molded product cannot be obtained. Recently, it has been found that a ring-opening polymerization of .epsilon.-caprolactone produces a higher molecular weight polymer, and proposed to use the polymer as a biodegradable resin. However, the resulting polymer is limited to only special applications because of a low melting point of 62.degree. C. and a high cost thereof. Further, although glycolic acid, lactic acid and the like are polymerized by a ring-opening polymerization of glycolide and lactide thereof to obtain polymers with higher molecular weights so as to be sometimes used as medical fibers and the like, the polymers are not used in great amounts as industrial parts, automobile members, domestic articles and the like because their decomposition temperatures are close to their melting point and they have defects in their molding properties.
Although most of these are applied to plastic injection-molded articles, it is no exaggeration to say that high molecular weight polyesters (referring to polyesters having number-average molecular weights of at least 10,000) generally used for the plastics are limited to polyethylene terephthalate, a condensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol. Although there are cases of 2,6-naphthalenedicarboxylic acid being used instead of terephthalic acid, there are no reports of trials which obtained polymers with biodegradability.
Therefore, it is safe to say that there has been no concept of trying to make the molded articles in practical use by injection molding using biodegradable aliphatic polyesters in which aliphatic dicarboxylic acid was used.
One of the reasons why this application concept has not been thought of is felt to be that in spite of the required special molding conditions and physical properties for the above injection-molded articles, most of the above-mentioned aliphatic polyesters have melting points of 100.degree. C. or lower even if they are crystalline, and have poor heat stability when melted above that. Of further importance is that the properties, particularly mechanical properties such as tensile strength, of these aliphatic polyesters show markedly poor values even when they are the same level of number-average molecular weight as the above-mentioned polyethylene terephthalate, so just conceiving that the molded articles having required strength and the like would be obtained was difficult.
Another reason seems to be that studies for improving the physical properties of the aliphatic polyesters by increasing their number-average molecular weights have not been sufficiently advanced because of their poor heat stability.
The object of the present invention is to provide injection-molded articles prepared by the above-mentioned aliphatic polyesters as their components which have sufficient high molecular weights for practical use and excellent mechanical properties represented by heat stability and tensile strength, and which can be decomposed by microorganisms and the like, i.e., are biodegradable as one means of waste disposal so that they may be easily disposed of after the use thereof.